The present invention relates to moving target indicator (MTI) radars, and more particularly to a doppler determination system for MTI radars employing an amplitude comparison of odd and even MTI functions derived from the same signal returns to determine the doppler frequency of a target return.
In radar systems using simple MTI waveforms, no estimate of target doppler is obtained. This makes it impossible to determine unambiguous range rate from the use of multiple pulse repetition frequencies (PRFs) and makes it impossible to do accurate angle estimation because of lack of knowledge of target amplitude when adjacent beams have different waveforms. As a result of this lack of knowledge, a three dimensional radar design may be required which uses redundant transmissions of different waveforms at the same angular position in order to estimate target angle. Further, radars may employ scan-to-scan PRF changes for velocity visibility rather than beam-to-beam or dwell-to-dwell because they could not otherwise determine angle from amplitude comparison between beams.
One class of 3 dimensional radar uses an antenna which rotates in azimuth while simultaneously generating a sequential scan of beams in elevation through either phase shifting or frequency scanning. The sequence of beams in elevation is called an elevation scan. The duration of an elevation scan is about the time that it takes the antenna to rotate through one azimuth beamwidth. The beams in elevation are spaced on the order of one elevation beam-width. The elevation of the target is measured by comparing the relative amplitude from adjacent beams which straddle the target position. This technique is called sequential lobing and is discussed in "Introduction to Radar System," M. Skolnick, McGraw Hill, 1962, pages 165-166. A simple estimate for the elevation angle is: EQU .theta..sub.e =K(log P.sub.1 -log P.sub.2)+.theta..sub.o ( 1)
where P.sub.1 and P.sub.2 are the return single pulse amplitudes from the target on beams 1 and 2, respectively, k is a constant having to do with the beam spacing and beam width and .theta..sub.o is the angle at the crossover between the beams.
Often MTI waveforms are used on the lower beams of the elevation scan to suppress clutter. Higher beams are above the clutter and MTI is not required. Range gated pulse doppler is seldom used in a radar of this type because it takes more time than a simple three pulse MTI which would reduce significantly the elevation coverage.
One problem that has occurred on radars of this type is the inability to make an angle measurement when the target is straddled by two beams, one of which is employing an MTI waveform and the other of which uses a single pulse. The problem here is that the equivalent single pulse amplitude, P, from the MTI waveform is unknown.
It is also sometimes desirable to estimate the radar cross section (RCS) of the target. Normally this can be done from the knowledge of the sensitivity parameters of the radar coupled with the knowledge of the range to the target and the return amplitude. This is of the form EQU RCS=kP.sup.2 /R.sup.4 ( 2)
where P is the single pulse return amplitude from the target, K is a constant having to do with the sensitivity of the radar and R is the range to the target as measured by the radar.
When MTI is employed, the equivalent single pulse amplitude is unknown, and it has heretofore not been possible to estimate the target RCS using an MTI waveform.
Often, the doppler frequency from a target will be ambiguous. This can occur when the range of potential target doppler frequencies is larger than the pulse repetition frequency (PRF). If the ambiguous doppler frequency is known, two or more PRFs may be used to determine the unambiguous doppler frequency and hence the range rate of the target. This is known as PRF switching and is described in "Introduction to Airborne Radar," George Stimson, Hughes Aircraft Company, 1983, pages 364-365. PRF switching is normally used with pulse doppler waveforms where an estimate of the ambiguous doppler may be made by comparing the amplitudes of the returns in adjacent doppler filters. This has not been useful to date for simple MTI waveforms because it was not possible to determine the doppler position of the return within the PRF interval.
It is therefore an object of the invention to provide a system for determining the doppler frequency (range rate) of the target return in a simple MTI radar system.
A further object of the invention is to provide a technique for estimating the equivalent single pulse amplitude of the MTI waveform and the elevation angle of a target detected by a 3-dimensional radar system which generates a sequential scan of beams in elevation when the target is straddled by two beams, one of which employs an MTI waveform, and the other of which uses a single pulse waveform or which uses an MTI waveform with different PRF.
Another object of the invention is to provide a means for estimating the radar cross-section of a target detected by an MTI waveform.